Full Length ArticleSynthesis of a novel magnetic Fe3O4/γ-Al2O3 hybrid composite using electrode-alternation technique for the removal of an azo dye
Graphical abstract
Introduction
With increasing industrialization, huge quantity of dye-containing wastewater is produced from various manufacturing industries such as food, rubbers, cosmetics, textile, paper, printing and others. Due to the ease of dissolution in water and resistance towards biodegradation, synthetic dyes have become one of the significant contributors to water pollution [1]. In particular, azo dyes and their metabolites have been recognized as toxic, mutagenic, as well as carcinogenic to the aquatic ecosystem and humans [2]. Consequently, a substantial increase in effort to develop sustainable means of dye containing wastewater treatment and improve the efficiency of the process is ongoing.
Numerous approaches have been unearthed and developed to treat dye-containing wastewater such as coagulation/flocculation, chemical oxidation, ultrasonication, adsorption, and so on. Among such techniques, adsorption process has evolved into one of the most promising methods due to its flexibility, simplicity of design, no release of harmful byproducts, and economical technique [3], [4]. In particular, metallic nano-sized materials, especially metallic nanoparticles, have recently gained widespread attention in the adsorption of dye molecules from solution phase due to its outstanding stability and adsorption capacity at low adsorbent dosage, as well as high productivity and cost efficiency [5], [6]. Out of the heavily investigated metallic nanoparticles, gamma alumina (γ-Al2O3) has been renowned as one of the promising candidates as an adsorbent because of its unique surface property (i.e., spinel structure), allowing favorable interaction with adsorbates, and thereby possessing a high potential to exhibit highly effective adsorption performance for various contaminants [7], [8], [9], [10], [11], [12]. Nonetheless, it is still considered to be an undesirable process with restricted applicability in industrial application due to its difficulty in separation without any post-treatment processes such as filtration or centrifugation, which may lead to the potential generation of secondary pollutants.
In this regard, use of nanoparticles in magnetic materials (e.g. Fe3O4, magnetite) offer tremendous potential as promising adsorbents for wastewater treatment than conventional (non-magnetic) adsorbents because of its synergistically improved properties and comfortable re-collectability and separability from aqueous solution by applying external magnetic field at the post-adsorption stage, which leads to improving the system’s attractiveness with respect to the simplicity and cost-effectiveness of the process [13]. Various methodologies have been utilized for synthesizing Fe/Al-based magnetic nanocomposites, which are mostly based on chemical synthetic method such as incipient-wetness impregnation [3], modified wet impregnation method [14], hydrothermal method [15], sol-gel method [16], and chemical precipitation method [17]. However, owing to its complexity, the physicochemical properties of final products would be unstable and un-reproducible, which ultimately may lead to inconsistent adsorption capacity.
Given the interest in developing an alternative route to synthesize magnetic nanoparticles, electrochemical synthesis using Fe electrodes has emerged as an alternative option for the fabrication of magnetite, specifically based on anode dissolution and electro-precipitation processes [18]. In this method, released iron from the anode is oxidized to ferrous ion (Fe2+), which is then further oxidized into ferric ion (Fe3+) in the presence of electrolytes. Subsequently, the generated hydroxyl ion (OH−) at the cathode via reduction of water molecules bind to the Fe3+ to yield Fe(OH)3. Finally, Fe(OH)3 is reduced to Fe3O4 under alkaline condition (7.0 < pH < 9.0). If OH− is not enough presented in the solution, it may dehydrate to generate a non-magnetic ferric oxide [19]. The overall reactions are as the following [20], [21]:Fe → Fe2+ + 2e−Fe2+ → Fe3+ + e−H2O → 2H+ + 2e− + 1/2O22H2O + 2e− → H2 ↑ + 2OH−Fe3+ + OH− → Fe(OH)33Fe(OH)3 + H+ + e− → Fe3O4 + 5H2O
Similarly, when an aluminum electrode is applied, Al3+ and OH− ions can be generated by electrochemical reactions at the anode and cathode, respectively. According to complex precipitate reactions, Al3+ is finally transformed into Al(OH)3, an important precursor for the thermal synthesis of γ-Al2O3. The reaction scheme of the process is as the following [22].Al → Al3+ + 3e−2H2O + 2e− → H2 ↑ + 2OH−Al3+ + H2O → Al(OH)2+ + H+Al(OH)2+ + 2H2O → Al(OH)3 + 2H+
In general, a rapid increase of pH of the system occurs during the intercourse of electrochemical synthesis due to the generation of OH− at the cathode such that the final pH may reach above 10.0 even when the initial pH is acidic. On the other hand, Al electrode acts neutralization neutralizer when the initial pH is alkaline [23]. This phenomenon serves as a potent tool in the synthesis of magnetic nanocomposites via electrochemical process because the consecutive usage of Fe electrode followed by Al counterpart can prevent the possible formation of unwanted ionized byproducts such as Fe(OH)4−. Taking these phenomena into account, the development of a novel approach to synthesize magnetic hybrid composite by the application of Fe electrode followed by Al electrode, or simply electrode-alteration technique, is investigated in this study. The specific objectives of the this work were to (i) synthesize magnetic Fe3O4/γ-Al2O3 hybrid composite (denoted as M-Fe/Al-H) by applying a novel electrode-alternation method and (ii) characterize its physicochemical properties as well as its potential application as an adsorbent for removing azo dye from aqueous media. To elucidate the adsorption mechanisms and behaviors of M-Fe/Al-H, kinetic and equilibrium isotherm studies were conducted under different temperatures. To our knowledge, no study has been reported on the application of electrochemically prepared M-Fe/Al-H as an adsorbent for the removal of AB1 from aqueous solutions up to date.
Section snippets
Chemicals and materials
To investigate the M-Fe/Al-H for dye adsorption, an amino acid staining di-azo dye acid black 1 (AB1, purity > 96.0%), which has a chemical formula of C22H14N6Na2O9S2 with a molecular weight of 616.49 g/mol was used as a model adsorbate, was purchased from Samchun Pure Chemical Co., Ltd. (South Korea). The chemical structure of AB1 is illustrated in Fig. S1 (see supplementary material). The stock dye solution was prepared by dissolving 1.0 g of AB1 in 1.0 L of deionized distilled water (Milli Q
Textural and morphological characteristics of M-Fe/Al-H
Textural and morphological properties of M-Fe/Al-H, including particle size distribution, XRD patterns, EDS spectra, N2 adsorption/desorption isotherm, and pore size distribution, are presented in Fig. 1. As depicted in Fig. 1(A), the particle size distribution of M-Fe/Al-H was measured in terms of number weighted mean particle size, indicating that the average particle size of the prepared adsorbent was 65.3 ± 2.4 nm with a relatively narrow distribution within the range of 44.5-91.8 nm. The
Conclusions
In this study, a novel M-Fe/Al-H has been developed via electrode-alternation technique followed by pyrolysis. Findings from preliminary test suggest that the prepared adsorbent possesses outstanding affinity for AB1 adsorption due to more desirable physicochemical properties than those of M-Fe. Adsorption kinetics revealed that the AB1 adsorption was rapid within the first 1.5 h and achieving over 90% equilibrium adsorption capacity within 3 h of contact time. The experimental data is in
Acknowledgement
This work was supported by “Advanced Technology Program for Environmental Industry” of the Korea Ministry of Environment (MOE) (No. E416-00020-0606-0).
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